1. Field of the Invention
The present invention relates to an error detecting device for detecting errors in received digital data such as voice data, image data, character data or control data handled on a frame by frame basis.
2. Description of Related Art
FIG. 8 is a block diagram showing a configuration of a receiver of a conventional digital portable telephone with an error detecting device of received digital data arranged in frames. In FIG. 8, the reference numeral 101 designates an antenna, 102 designates an RF stage for accepting a received signal through the antenna 101, 103 designates a demodulator, 104 designates a Viterbi decoder, 105 designates a CRC (Cyclic Redundancy Check) decision portion as an error detector, 106 designates a voice decoder, 107 designates a D/A (Digital/Analog) converter, and 108 designates a speaker, which are connected in this order.
Next, the operation of the conventional device will be described with reference to the flowchart of FIG. 9.
Received digital data obtained through the antenna 101, RF stage 102 and demodulator 103 is fed to the Viterbi decoder 104 which carries out error correcting of a voice frame or the like at step ST7-1. Subsequently, the CRC decision portion 105 carries out at step ST7-2 error detection of only most important bits which will be described later. If a decision is made that the received digital data is correct, the voice decoder 106 performs decoding at step ST7-3, and operates the speaker 108 through the D/A converter 107. In contrast, if a decision is made that the received digital data is incorrect, the voice decoder 106 carries out post-error-detecting processing such as mute processing of the voice data at step ST7-4, in which case the D/A converter 107 does not operate the speaker 108.
With such a configuration, the conventional error detecting device of the received digital data performs error detection of only the most important bits after the Viterbi decoding. It sometimes, however, misses detecting a considerable number of errors occurring in the most important bits, resulting in odd sounds produced from the speaker 108 through decoding of the voice data. For example, with regard to the full rate voice frame of the GSM (Global System for Mobile Communications) system, the CRC portion misses many errors occurring in the most important bits because it performs the error detection on the basis of parity bits consisting of only three bits, and hence its error detecting probability is rather low.
The full rate voice frame in the GSM is 20 ms in length, and consists of 260 information bits which are divided into important 182 class-1 bits which undergo error correction, and 78 class-2 bits which do not undergo the error correction. The 182 class-1 bits are further divided into the most important 50 class-1a bits and 132 class-1b bits, and only the most important 50 class-1a bits are subjected to the error detection, that is, the CRC decision.
Thus, the CRC decision portion 105 passes the information bits through a divider to make a decision that no error has occurred if the remainders in the transmitter and receiver match each other, or that some error has occurred if they do not match, in which case the mute processing is carried out. Sometimes, however, because of an error of the parity bits themselves during the transmission, the CRC computed by the receiver can match the CRC generated by the transmitter, even if they must be actually different from each other. In this case, the odd sounds result from the decoding of the voice data.
The probability of such erroneous detection depends on the number of bits of the parity bits of the CRC decision portion 105. For example, when the parity bits consists of three bits, the probability of making a wrong decision becomes 1/8=12.5% in the worst case.